Marine vessels or ships are usually equipped with D.C. power generators for powering the various electrical and/or electronic devices aboard the vessel from a D.C. power bus. It is characteristic of such marine D.C. power sources that they include voltage spikes superimposed on the power lines, as well as radio frequency (R.F.) noise, or abrupt input voltage drops along with momentary losses of power and complete losses of power. A drop in input voltage to a powered device is typically caused by engine cranking. The present day vessels generally have sophisticated electronic equipment on board, such as navigation electronics or the like. These days the navigation electronic equipment may be LORAN navigation receivers or satellite navigation receivers. Other sensitive electronic shipboard equipment may be video plotters and GPS equipment. These shipboard electronic devices, when powered from a source of D.C. that includes the aforementioned electronics signals or noise thereon are considered "unclean" D.C. power and will cause improper operation or malfunction of the electronic devices to which they are coupled. For example, the navigation electronic equipment is very sensitive to input voltage spikes, surges in power, brownouts (lower input voltages), abrupt voltage changes and electronic noise interference, which is common to the marine power sources. When navigation equipment is powered from such a marine power source, these varying power conditions may cause memory loss, inaccurate output navigation data, component failure, or general malfunction of or damage to the navigation equipment. Accordingly, such sensitive marine electronic devices require a "clean" or "pure" source of power without the aforementioned common irregularities on the power lines for proper operation. In the event of a momentary drop in voltage or loss of power from the D.C. source, the vessel operator may not be aware of the interruption of the operation of his onboard electronic equipment. Accordingly, there is a present need for protective circuits for marine electronic equipment that is powered from such marine D.C. power sources that eliminates the above-mentioned "impurities" from the D.C. power applied thereto and also provides a back-up or standby power source to power the electronic equipment in the event of power loss and/or drop of voltage level from the power source. In addition, some signalling means is desired to make the vessel operator aware of the fact that he has lost power to his electronic devices and the electronics are operating on the basis of a back-up power source for permitting him to take corrective action by either restoring the power to the electronic equipment or turning off the electronic devices before the complete loss of power to the equipment causes a problem, i.e., the voltage from the back-up source drops or is minimal for proper operation.